The present invention generally relates to a line access apparatus for in-band communication and, in particular, relates to such an apparatus including a high voltage isolating coupler and low voltage circuitry to interconnect a plurality of subscriber line pairs with a communication network.
The introduction of and continued demand for non-voice communication services is well recognized. As a result, entire non-voice communication networks have been developed to serve both the individual consumer and large users. In general, the provision of such services have taken two tacks. One tack being the provision of a separate and distinct communication system, the other tack being the expanded use of existing forms of communication. The provision of separate and distinct communication systems is, of course, expensive as it requires, inter alia, complete new wiring, whereas the use of existing forms of communication, most frequently telephone lines, requires techniques that do not interfere with existing voice services. The line access apparatus for in-band communication described and discussed hereinafter is directed to the latter tack.
Currently, the essential problems encountered when attempting to use existing telephony systems are two-fold. The first problem is access and the second problem is the cost and reliability of equipment capable of functioning at the relatively high voltages required of telephone line circuits.
Existing telephony systems are composed of a number of switches, each switch serving a large number of telephone subscribers. These switches are geographically distributed and are generally referred to as "central office switches". Each central office switch has a main distribution frame (MDF) whereat each subscriber line pair served by that switch enters one side thereof. The subscriber line pairs are then routed through the MDF to the switch mechanism. This configuration is also found in smaller, usually privately owned, switches serving a rather geographically concentrated group of subscribers, such as, for example, a university campus. These smaller switches are generally referred to as private branch exchanges (PBXs). Regardless of the size, most telephone company switches are presently operating at near peak capacity. Because of the near peak capacity it would be impossible to provide each subscriber with a second pair of wires for non-voice communication via the MDF. Even if the capacity were available, the cost of the existing wiring militates against such an approach and massive disruption of existing voice communication would most likely occur. However, to make efficient use of existing communication lines, the MDF is clearly a most advantageous location to access a large number of subscriber lines. It is primarily for this reason that alternatives to rewiring an MDF have been developed for accessing existing subscriber lines.
One particular development is shown and described in U.S. patent applications Ser. Nos. 595,108 and 595,120 both filed on Mar. 30, 1984 and assigned to the assignee hereof. These applications are deemed incorporated herein by reference. In these referenced applications a main distribution frame access device is discussed and described. The MDF access device typically includes a plug-in unit that is inserted at the main distribution frame between the subscriber line termination points and the high voltage protection points. The use of such a device can provide immediate access to the subscriber pairs of wires within the main distribution frame without disturbing or disrupting the internal wiring thereof.
Such an access device is particularly useful in systems wherein additional services are to be provided via the subscriber lines, for example, for providing an auxiliary data switch or providing a remote meter reading system or other telemetry applications. Telemetry systems have been described and discussed in U.S. patent applications Ser. Nos. 648,542; 667,527 and 667,789 filed on Sept. 7, 1984, Nov. 2, 1984 and Nov. 2, 1984, respectively. These applications are assigned to the assignee hereof and deemed incorporated herein by reference. In such systems the main distribution frame access device can be used to provide access to the subscriber lines and connected, via a multiplexing means, to the data processing equipment of a utility company.
The second problem is implementing systems that make use of existing communication means is that telephone subscriber line circuits are designed to sustain comparatively high voltages, such as, for example, a -48 volts D.C. commonly used in the talking circuits thereof, as well as a superimposed 75 RMS A.C. ringing voltage. This voltage is conventionally provided by the central office battery and ringing generator, and exists intermittently on the subscriber lines when rung in response to a telephone call made thereto. In addition, for obvious reasons, telephone subscriber line circuits are also designed to survive lightning strikes and other high voltage transients.
The fact that such line circuits are subjected to and must sustain relatively high voltages is a detriment to adding auxiliary services at the MDF via a main distribution frame access plug. For example, when multiplexing/demultiplexing is performed at the MDF, high voltage large scale integrated circuits are presently required.
Currently, high voltage large scale integrated circuits, in addition to being expensive compared to low voltage large scale integrated circuits, are somewhat limited in both the manufacture and reliability thereof. Furthermore, currently available high voltage large scale integrated circuits are, themselves, extremely susceptible to high voltage transient signals and could exhibit catastrophic failure due to transients occurring at, or near, the main distribution frame.
Further, if an existing communication media has loaded or long subscriber lines the operating bandwidth thereof is often quite restricted. In the instance of telephone communication systems, the bandwidth is limited to the voice band. This places a further retriction on any auxilliary communication system in tandem therewith. Hence, to implement a system using such existing communication media the communication must be made within the operating bandwidth thereof, i.e., in-band.
Consequently, in order to take full advantage of the ability to access subscriber line pairs at a main distribution frame, it is highly desireable to isolate high voltage signals while, nevertheless, exchanging information with the subscriber lines.